The Fischer-Tropsch (F-T) process for making alcohols dates to the 1920s. Work in this field up to the late 1970s was reviewed by Anderson, et. al. (R. B. Anderson, J. Feldman and H. H. Storch, Ind. Eng. Chem., 44 (1952), 2418; R. B. Anderson, Catal. Rev., 21 (1980) 53.). The F-T process involves passing synthesis gas (or syngas, a mixture of carbon monoxide and hydrogen) over a catalyst to form alcohols. The Anderson papers list a number of catalysts for making alcohols in this way, including those containing zinc, copper, chromium, manganese, thorium and iron. Many times these are promoted with alkali metal salts.
Syngas is made by gasifying carbon-containing materials (such as coal, plant or animal-based biomass, petroleum-based hydrocarbons, natural gas or municipal solid waste) according to the following general equation:CXHYOZ→XCO+YH2 where X≧1; Y≧0, Z≧0. This reaction is carried out under high heat conditions. If the starting material is rich in carbon (such as coal or coke) oxygen or steam are used as reactants. In order to get the correct ratio of hydrogen (H2) to carbon monoxide (CO) the water gas shift reaction is usually employed:CO+H2O→H2+CO2 The optimum H2/CO ratio depends on the catalyst and temperature employed and the products desired. This reaction is usually carried out in a separate reactor with catalysts other than the F-T types.
In the 1980s, Dow discovered that molybdenum sulfide was preferred versus the previous art. The advantages include: (1) The high water-gas shift activity allowed the use of carbon monoxide-rich syngas without installing a separate water-gas shift reactor. (2) High yields and selectivities for higher alcohols were realized. The MoS2 catalyst required alkali promoters and a cobalt co-catalyst to obtain the best results. (G. J. Quarderer and G. A. Cochran, “Process for producing alcohols from synthesis gas”, U.S. Pat. No. 4,749,724 (Jun. 7, 1988); R. R. Stevens and M. M. Conway, “Mixed alcohols production from syngas”, U.S. Pat. No. 4,752,623 (Jun. 21, 1988); R. R. Stevens and M. M. Conway, “Mixed alcohols production from syngas”, U.S. Pat. No. 4,831,060 (May 16, 1989); R. R. Stevens, “Process for producing alcohols from synthesis gas”, U.S. Pat. No. 4,882,360 (Nov. 21, 1989)). Typical temperatures and pressures were 250 to 330° C. and 1500 psig. The Dow patents disclosed that alkali-promoted molybdenum carbide is a catalyst for alcohol production but gave no examples and stated that MoS2 is the most preferred catalyst. The Dow patents also disclosed the use of nickel as a co-catalyst but gave no examples with Mo2C.
Most recently, Lee and co-workers (H. C. Woo, K. Y. Park, Y. G. Kim, I-S Nam, J. S. Chung and J. S. Lee, Applied Catalysis, 75 (1991), 267; J. S. Lee, S. Kim and Y. G. Kim, Topics in Catalysis, 2 (1995), 127) have used alkali promoted, high surface area molybdenum carbides to produce higher alcohols from syngas at about 300° C. and pressures as low as 145 psig. Two practical methods have been described for synthesizing high surface area Mo2C. One involves a Temperature Programmed Reduction (TPR) of a metal salt with a fluid stream. (J. S. Lee, S. T. Oyama, M. Boudart, Journal of Catalysis, 106 (1987), 125) The other requires the reaction of a vaporized metal salt with a solid substrate. (M-J Ledoux, J-L Guille, C. Pham-Huu and S. Marin, “Production of heavy metal carbides of high specific surface area”, U.S. Pat. No. 5,308,597 (May 3, 1994). M-J Ledoux, J-L Guille, C. Pham-Huu and S. Marin, “Production of heavy metal carbides of high specific surface area”, U.S. Pat. No. 5,391,524 (Feb. 21, 1995).) Boudart, et. al. (J. S. Lee, S. T. Oyama, M. Boudart, Journal of Catalysis, 106 (1987), 125; J. S. Lee, L. Volpe, F. H. Ribeiro, M. Boudart, Journal of Catalysis, 112 (1988), 44) were the first to disclose a procedure for producing high surface area, catalytically active Mo2C. The reactants were powdered MoO3 which was carburized with a methane/hydrogen or ethane/hydrogen mixture using the TPR (Temperature Programmed Reduction) procedure. TPR is a procedure for gradually deoxygenating and carburizing the MoO3 powder using a programmed heating rate. The TPR parameters were determined so as to produce mainly active, crystalline Mo2C hexagonal close-packed crystals. The final temperature had to be high enough with the proper heating rate to produce active Mo2C crystals. If the final temperature was too high or the heating rate too slow at high temperatures, a non-carbidic carbon layer would cover the catalyst and deactivate it. The TPR process appears to be most easily scaled-up to production levels.
The driving force for the work by Dow Chemical and Lee, et. al. was to produce an alcohol mixture suited to blend with gasoline. It is therefore desirable that the alcohol mixture should approximate the boiling range of gasoline with minimal purification. An efficient process would have high mass yields of mixed alcohols per unit mass of catalyst with high selectivity towards C2 to C5 alcohols. The main variables controlled during alcohol synthesis were: temperature, pressure, space velocity and reactant gas ratio (H2/CO). Space velocity (SV) reflects the rate of reactant gas going through the reactor per unit of catalyst. Herein it is expressed as liters of CO+H2 per hour per kg of catalyst. Product yield is expressed as total grams of alcohol produced per hour per gram of catalyst. One measure of selectivity is grams of higher alcohols (C2OH+), particularly ethanol (C2OH), produced per hour per gram catalyst. When compared to total alcohol yield, this measure of selectivity expresses the selectivity towards higher molecular weight alcohols than methanol. C2OH+ refers to all alcohols with molecular weights greater than methanol. Higher selectivity is more desirable because high methanol concentrations give lower boiling ranges than desirable for mixing with gasoline and can lead to engine corrosion.
It would be desirable to develop a catalyst that could produce a similar product as the Mo2S, but that did not contain sulfur. With regulations for sulfur becoming more stringent, it seems reasonable that such a catalyst might be valuable because there would be no possibility of making sulfur-containing byproducts (which is a distinct possibility when using a sulfur-containing catalyst).
It would also be desirable for the yields to increase with space velocity without sacrificing selectivity towards higher alcohols. Indeed, such is a goal of at least one embodiment of the inventive technology.